Michael Rosenbaum spoke out about his former co-star Allison Mack’s alleged involvement in NXIVM. (Getty Images)

Allison Mack’s former “Smallville” co-star Michael Rosenbaum said the actress’ involvement with alleged sex cult NXIVM is “shocking” and “surprising” to him, but recalled her once mentioning an organization that he felt was a “little culty.”

Rosenbaum, 46, who played Lex Luther and acted alongside Mack for seven seasons of the superhero show, said hearing the news about his former co-star made him realized he didn’t know the actress all that well.

“It’s like someone telling you that your brother murdered someone,” Rosenbaum said on the podcast “This Past Weekend.” “You’re like, ‘No, he didn’t. I know my brother. He wouldn’t kill anybody.’ It’s just so bizarre. The whole story I think is yet to come out. I think it’s just shocking and surprising.”

The cast of “Smallville,” where Michael Rosenbaum and Allison Mack starred alongside each other for seven seasons. (Warner Bros./Facebook)

Rosenbaum recalled Mack as the “sweetest, most professional” woman while working on “Smallville” from 2001 to 2008, when the 46-year-old actor left the series. He said he “really didn’t hang out” with Mack off-set during the seven years they worked together, but they always had mutual respect for one another.

“She was just a great girl, great actress. And I remember [her] doing some self-help stuff. I remember her mentioning something. An organization. And I didn’t really listen. I just remember thinking, ‘Oh, that’s a little culty. That’s not for me,’” he said.

The actor continued, “She was like a lot of actors and actresses. She wanted attention. She wanted approval, and she found something that she probably thought would give her that. Ultimately, inadvertently, she got into something that was bigger than her.”

Mack was arrested in April and faces charges of sex trafficking and conspiracy to commit forced labor stemming from her time with NXIVM. The 36-year-old actress joined the alleged sex cult in 2006 and rose in the organization’s ranks to become a so-called “slave master” who allegedly recruited sex slaves under the guise of a women’s self-help organization. She worked alongside NXIVM founder Keith Raniere and other top members.

She was apparently looking for spiritual guidance in her life when she bumped into the cult, according to a Hollywood Reporter exposé.

Rosenbaum said he never imagined Mack being involved in a cult.

“I thought she would be married with kids. She was smart, and her family was into music and opera and stuff, and sort of upper class,” he said. “I don’t know what that means, but she had her stuff together.”

“You just don’t ever really know someone,” he said.

Mack isn’t the only former “Smallville” actress linked to NXIVM. Actress Kristin Kreuk, who played Lana Lang in the series, was also accused of being involved in the group. She said in late March she took a NXIVM course when she was 23 and left the organization five years ago. She denied recruiting sex slaves or knowing about the organization’s alleged criminal activity.

Katherine Lam is a breaking and trending news digital producer for Fox News. Follow her on Twitter at @bykatherinelam